linux-kernel - Re: [RFC PATCH] rust: types: Add explanation for ARef pattern

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <beaf1fa3-eebe-443c-bc51-abd9348a411d@proton.me>
Date: Fri, 26 Jul 2024 13:43:38 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, Jonathan Corbet <corbet@....net>, Viresh Kumar <viresh.kumar@...aro.org>, Danilo Krummrich <dakr@...hat.com>, Trevor Gross <tmgross@...ch.edu>, gregkh@...uxfoundation.org
Subject: Re: [RFC PATCH] rust: types: Add explanation for ARef pattern

On 25.07.24 22:29, Boqun Feng wrote:
> On Thu, Jul 25, 2024 at 06:12:56PM +0000, Benno Lossin wrote:
>> On 24.07.24 19:44, Boqun Feng wrote:
>>> On Tue, Jul 23, 2024 at 11:14:29AM +0200, Alice Ryhl wrote:
>>>>> +/// However `&Self` represents a reference to the object, and the lifetime of the **reference** is
>>>>> +/// known at compile-time. E.g. the `Foo::as_ref()` above.
>>>>> +///
>>>>> +/// ## `impl Drop` of an `impl AlwaysRefCounted` should not touch the refcount
>>>>> +///
>>>>> +/// [`ARef`] descreases the refcount automatically (in [`ARef::drop`]) when it goes out of the
>>>>> +/// scope, therefore there's no need to `impl Drop` for the type of objects (e.g. `Foo`) to decrease
>>>>> +/// the refcount.
>>>>>  pub struct ARef<T: AlwaysRefCounted> {
>>>>>      ptr: NonNull<T>,
>>>>>      _p: PhantomData<T>,
>>>>> --
>>>>> 2.45.2
>>>>>
>>>>
>>>> I think this is missing some basic information related to `&Self` ->
>>>> `ARef<Self>` conversions. We should explain that these conversions are
>>>> possible, and that you usually don't want `raw_ptr` -> `ARef<Self>` to
>>>> increment the refcount - instead provide a `raw_ptr` -> `&Self` and
>>>> convert the `&Self` to `ARef<Self>`.
>>>>
>>>
>>> I could be more explicit on this, but could there be a case where a `T`
>>> only wants to return `ARef<T>` as a public API? In other words, the
>>> author of `T` doesn't want to expose an `-> &T` function, therefore a
>>> `-> ARef<T>` function makes more sense? If all the users of `T` want to
>>> operate on an `ARef<T>` other than `&T`, I think it makes sense, right?
>>
>> You can always get a `&T` from `ARef<T>`, since it implements `Deref`.
>>
> 
> Yeah, but this is unrelated. I was talking about that API providers can
> decide whether they want to only provide a `raw_ptr` -> `ARef<Self>` if
> they don't need to provide a `raw_ptr` -> `&Self`.
> 
>>> Overall, I feel like we don't necessarily make a preference between
>>> `->&Self` and `->ARef<Self>` functions here, since it's up to the users'
>>> design?
>>
>> I would argue that there should be a clear preference for functions
>> returning `&Self` when possible (ie there is a parameter that the
> 
> If "possible" also means there's going to be `raw_ptr` -> `&Self`
> function (as the same publicity level) anyway, then agreed. In other
> words, if the users only need the `raw_ptr` -> `ARef<Self>`
> functionality, we don't want to force people to provide a `raw_ptr` ->
> `&Self` just because, right?

I see... I am having a hard time coming up with an example where users
would exclusively want `ARef<Self>` though... What do you have in mind?
Normally types wrapped by `ARef` have `&self` methods.

Cheers,
Benno